1. Field of the Invention
The present invention relates to a process for manufacturing SiC-based ceramic precursors by curing a polycarbosilane shaped body, particularly a granular, powdery, fibrous, filmy or other finely shaped body.
2. Description of the Prior Art
Ceramics have drawn attention as a material excellent in heat-resistance, abrasion-resistance, high-temperature strength or the like. Diversified materials have been molded and sintered by various processes and used as ceramic materials. A process for synthesizing ceramic materials, inter alia, wherein polymers containing a metallic element, beginning with organosilicone polymers, are used as a ceramic precursor, is particularly suited for manufacturing ceramic materials, such as fibers, which are impossible to manufacture according to processes wherein powder is molded and sintered. Accordingly, various polymers have so far been studied. Among these polymers, polycarbosilanes having a skeleton composed mainly of SiC linkages are very useful as a precursor for SiC-based ceramics. Particularly, silicon carbide fibers synthesized by melt-spinning a polycarbosilane followed by curing and then firing have been extensively studied for application as a fiber reinforcement in composite materials.
Heretofore, in the case where such polycarbosilanes are employed as a ceramic precursor, there have been used processes wherein a polycarbosilane is melted or dissolved in a solvent, then shaped into a desired shape and fired to yield a ceramic. The shaped body is generally subjected to a curing treatment before firing so as not to melt in the firing step after shaping. There may be the case where firing can be conducted without the curing treatment. However, it is in the case of polymers of a high molecular weight pyrolyzable without passing through a molten state. Such a polycarbosilane is synthesized in a low yield and requires a dry-spinning or dry-extruding process for fiber- or film-formation, so that it is lacking in industrial value. As a generally employed curing treatment process, processes for heating polycarbosilanes at a temperature in the vicinity of a softening point thereof or irradiating with .gamma.-ray, electron beams or the like, in an oxidative atmosphere such as air or oxygen, are disclosed in Japanese Patent Laid-open Application No. 52-5,321 and Japanese Patent Publications Nos. 58-22,570 (J. P. No. 1,194,004), 61-9,246 (J. P. No. 1,345,083), and 62-14,647 (J. P. No. 1,408,310). Alternatively, as a process for curing polycarbosilanes without introducing oxygen thereinto, a process of irradiating electron beams in vacuum or an inert gas atmosphere is disclosed in Japanese Patent Publication No. 62-14,647 (J. P. No. 1,408,310) and further recently in Polymer Preprints, Japan, Vol. 38, No. 12 (1989).
However, amongst these processes, the process of treating polymers in an oxidative atmosphere, since oxygen is introduced into the polymers by oxidation, causes deterioration of characteristics of synthesized SiC-based ceramics. Further, high-temperature strength remarkably decreases, since the polymers are crystallized into cristobalite at high temperatures or emit carbon monoxide decomposed at further higher temperatures.
Alternatively, the curing process without introducing oxygen wherein electron beams are irradiated in vacuum or an inert gas atmosphere has drawbacks such that special apparatuses are required for electron beam irradiation, etc., treatment on a massive scale is difficult to conduct, or the like.